Balancing and filter system for radioreceivers



Nov. 19, 1935. w ET AL 2,021,676

BALANCING AND FILTER SYSTEM FOR RADIORECEIVERS Filed Jan. 10, 1955 2 Sheets-Sheet l JUL/51770.75. 7. 25572270270 0 Z/Z/ggad gmuzz $70 51'',

Nov. 19, 1935. R. E. WOOD ET AL 6 BALANCING AND FILTER SYSTEM FOR RADIORECEIVERS Filed Jan. 10,;1935 2 Sheets-Sheet 2 12 2125177275. HaymaniEIZ/Z/b ad. 5 Bag 11201255 504 7 Patented Nov. 19, 1935 BALANCING AND FILTER SYSTEM FOR RADIORECEIVERS Raymond E. Wood and Raymond S. Yoder, Chicago, Ill., assignors to Galvin Manufacturing Corporation, Chicago, 111., a corporation of Illinois Application January 10, 1935, Serial No. 1,202

21 Claims. (Cl. 250-20) Our invention relates to systems for eliminating ignition interference from being introduced into radio receiving sets employed in automobiles, aeroplanes, and. the like, an objects being to provide an improved system of the above character adapted for use with radio receiving sets in automobiles, aeroplanes and the like, the various features and advantages of our improvements being more particularly referred to in the ensuing part of the specification and appended claims.

In automobile radio receivers the ignition interference has presented a serious problem. In order to have clear reception and reasonable sensitivity in the receiver it has ordinarily been necessary to install suppressors on the engine spark plugs of the automobile. This increases the cost of the receiver installation and acts to reduce the operating speed range of the automobile.

It is an object of our invention to provide means for eliminating ignition interference in the radio receiver without impairing the operating speed of the automobile.

A further object is to provide an inexpensive device for use with the automobile radio receiver for accomplishing the above result.

One of the features of our invention is the provision of a combination of a balancing system and a filter system effective over the usual broadcast range. In general we employ an arrangement in which the balancing portion effectively cancels ignition interference in the antenna system of the receiver by introducing an ignition interference voltage of equal amplitude, substantially same wave shape, and of opposite phase to that ignition voltage impressed on the regular receiving antenna, said balancer being most effective over the higher frequency half of the broadcast band where the reflected impedance in the antenna system from the first tuned circuit is high. In combination with this balancer circuit we are enabled to employ a filter to great advantage as the filter may be designed to attenuate the ignition interference more effectively over the lower half of the broadcast band frequency spectrum. With this arrangement we are enabled to attenuate ignition interference over a wide carrier frequency range as certain shortcomings of each portion of the system are compensated by the other.

Another feature of our invention is the utilization of the standing wave voltage that exists on the car chassis for balancing ignitioninterference in the automobile radio receiver.

connected to the auxiliary or balancing, of ig- Although in the preferred form of our system we employ the combination of the balancing system and filter system, it is to be understood that either one may be used for some purposes without the other, and that there are also other features of our invention which will be more particularly pointed out.

For a better understanding of our invention, reference is to be had to the accompanying drawings, in which:

Fig. l is a diagrammatic circuit arrangement showing the antenna system connected to the antenna coil of the radio receiving set.

Fig. 2 is a modified view of the balancing portion of the circuit of Fig. 1.

Fig. 3 is a modified form of the balancing circuit with the balancing or auxiliary antenna coupled to the distributor lead.

Fig. 3a is a modified form of the balancing circuit utilizing one of the parts of the 'car.

Fig. 4 is a side elevation of one of the inductances employed in the various coils of the circuit with the turns expanded.

Fig. 5 is a right end view of Fig. 4.

Fig. 6 is a top plan view of the apparatus of 25 the balancing and filter circuit apparatus.

Fig. 7 is a right end view of Fig. 6.

Fig. 8 is a front view of Fig. 6, but with the front wall of the casing cut away.

Fig. 9 is a sectional view along the line 9-9 of Fig. 6 showing more particularly the electrostatic shield arrangement positioned between the coils of the balancer circuit; and I Fig. 10 is a detail view of the electrostatic shield removed from its support.

Referring now more in detail to the circuit arrangement as shown in Fig. 1, we provide a balancing circuit comprising a pair of relatively adjustable coils l5--l6 having an electrostatic shield l1 interposed between the coils, the coils and shield being preferably placed within a shelded compartment I8. One of the coils as I5 is connected with the broadcast receiving antenna l9, while the other coil l6 of the pair is mutual inductance is varied by changing the relative position of the coils in order that the voltage induced into coil I5 and the main antenna receiving circuit may be adjusted until equal to that of the ignition voltage interference induced through the main antenna. I9 into this same coil I5 and its associated circuit. The static shield i1 placed between the windings I5-I6 is preferably so designed and arranged to reduce the capacity coupling between these windings for phase changing and control of ignition energy transfer.

The main antenna I9 is usually placed in the roof of the car, although not limited tothat position, and passes through a shielded lead-in casing 2| preferably grounded to the metallic shielding enclosure of the apparatus which in turn is grounded to the chassis of the car in the installation of the apparatus. There may be an additional ground connection to the chassis, however, from the shielded casing 2| by a connection as indicated at 25 near the main antenna end of the casing to a corner post, the dash, or the side of the car.

The balancing antenna 23 is connected through a lead-in conductor 22 insulated throughout its length and passing through a metallic shielding covering 23 grounded to the shielding container. The metallic shielding 23 is also insulated throughout its length to prevent ignition interference from being introduced into conductor 22 by grounds on shield 23, inasmuch as such in: terference may be of undesired phase.

Interference antenna 20 is shown located under the hood 24 of the car or near the engine and ignition compartment Whether on a car, an aeroplane, or similar conveyance. However, this antenna 20 may be the control rod, some wire cable, chassis, metallic part of the body at a point near car wiring leads, cables, braces, supports or pipes positioned where they will pick up the ignition currents. That is, in the present invention we have found that we may utilize the standing wave voltage in metal parts of the car chassis which are not under the hood, for balancing the ignition interference in the receiver.

As stated, either a free antenna or a part of the car equipment may be employed for the interference antenna, and if a free antenna is used, it should be practically shielded from the roof or other antenna. as I9, and such is the case when this interference antenna is placed in the motor compartment 24. However, when a control rod or some other part as previously enumerated, is used for the interference antenna, we have found that complete shielding of the interference antenna is not necessary when the car chassis ignition currents are used for ignition interference input to balance the input of ignition interference to the broadcast receiving antenna.

The cables 26 and 22 of the antenna input system have nearly the same characteristic impedance, varying from '70 to 150 ohms. The terminating impedances for both systems are of the same order as the characteristic impedances, and the ratio between the two as nearly unity as practicable. With these precautions, the phase angle error between the existing and induced ignition interference voltages on the antenna primary is negligible. The same wave shape for transients in both systems if picked up from the same radiating source is obtained by the above precaution and by the proper choice of the interference antenna as well as its location.

Considering the filter circuit, we preferably employ a suitable type of low pass or band pass shielding container.

filter comprising several sections each independently shielded as at 33 and 3| with the choke or inductance coils 3233 thereof connected in series with the winding I5 of the broadcast input circuit and also in series with the primary wind- 5 ing 34 of the antenna coil. Shunt condensers 3536 form part of the filter circuit, one plate of each of which is connected to the grounded The filter and balancing compartments of the shielding container are pref- 10 erably separate and all suitably grounded and thus forming one side of the circuit to the grounded end of the primary of the antenna coil, the condensers of the filter, and the grounded end of the eliminating winding I6 of the balancer 15 transformer.

The various coils I5, I6, 32 and 33 are preferably of low loss design having a high the windings being of low resistance (D. C.) and low distributed capacity, the flat helical type of winding being preferred. Condensers 35 and 36 of the filter are low loss capacitors with short 25 leads to reduce inductance. Windings I5, I6, 32 and 33 of the character shown in Figs. 4 and 5 consist of approximately five turns of No. 14 enamel wire, and each coil 2.5 uh.

The radio receiver circuit is indicated at R and is preferably enclosed in a separate shielded container 39, the antenna coil being shown with the primary winding 34 of low impedance with a very narrow width of winding, preferably lattice-type, on a sleeve slipped over the secondary 43. An electrostatic shield 4| may be positioned between the windings.

With the circuit connected as shown in Fig. 1, the broadcast receiving circuit is indicated as passing from the antenna I9 through the balanc- (.0 ing and filter circuits to the primary of the antenna coil. The other side of the circuit is grounded to the shielded container, a common ground being provided for the antenna coil pri- 7 mary 34, and condensers 35 and 35. The con- 45 nection of this common ground, if through the chassis and apparatus housing, must be of low resistance and as non-inductive as practical.

In Fig. 2 we have shown a modified form of the balancer circuit in that a loading inductance 50 42 is connected between the balancer coil I6 and ground. Such an inductance may be provided so as to balance with the broadcast input circuit. Theoretically the impedance and phase of the voltages of the broadcast antenna and circuit, and the auxiliary or balancing antenna and its coilslii should be the same, and at times it might be desirable to provide a loading inductance 42 to this end. In practice, considerable variation in values as between the two antenna circuits is 60 permitted Without harmful effects. When such an inductance 42 is provided it should be magnetically isolated from the balancer coils I5I6.

In Fig. 3 we have shown another modified form of the circuit in which the auxiliary or balancing 55 antenna 20 is coupled to the distributor lead A by capacity coupling so as to take up the ignition currents there.

In Fig. 3a we have shown a further modification in which the balancing circuit is shown dia- 70 grammatically in combination with the throttled rod B of the automobile. In the construction we utilize the standing wave voltage always present in the car to provide the counter-ignition interference voltage. Although we have shown this 75 modification with. the: connection to the. throttle rod, it understood. that the. standing wave. volt:- age may be taken from. other parts of the car. Eurthermore, we have found that. the connection between the balancing circuit and the partof. the car; need not be under the enginehood.

An electronic device containing a cathode, con.- trol grid and anode is actuated by the potential difierence applied between the control grid. and cathode.. With the cathodeof the input tube of the receiver grounded to, the metal portionso-f the car, aircraft or other conveyance in the usual connection, and thus common to said portions from the standpoint of radio frequency voltage, the metal portions cannot be used as an inter.- ceptor of. signal energy because no potential would bedeveloped between the controlgrid and the. cathode. That is, from the standpoint of signal, there is no potential. In the case of interference voltage, however, all metal parts common to the interference source are emitters of that energy, and a standingwave voltage on these members is sufiicient to utilize for balancing. It is understood, of course, that the interference potential may vary over these metal portions. The interference radiates from the metal portions of the car to the signal antenna. rather than directly from the source under the hood, hence we can utilize it in interference elimination in the antenna system. As to the frequency. of this interference in a car, aircraft, or the like, this is beyond the received frequency spectrum. Therefore we accomplish elimination of interference without attenuation of signal energy in the circuits of Figs. 1, 2, 3, and 3a because we use a balancing potential free from signal energy, and a low-pass filter which cuts off frequencies beyond the receiver frequency spectrum.

Referring now to the balancing and filter circuit apparatus as shown in Figs. 6 to 10, the parts are all suitably shielded in iron shielding compartments as indicated diagrammatically in Fig. 1. As to the balancing apparatus, I provide a pair of relatively adjustable coils l-l6,. the winding of coil l5 consisting of enameled wire formed as indicated in Fig. 4.- and then pressed together and held within an insulating casing consisting of an outer cup portion 43 of fiber and a closing disc 44 also of sheet fiber, and mounted upon a wooden insulating post 45. Thus the coils I5-l6 are in the nature of an adjustable air core transformer with the coil I6 mounted upon a plate 46 carrying two threaded hubs 41--48 through which the rotatable screws 4950 are threaded. These two screws are interconnected by a large gear 5| meshing with the pinions 52-53. The screw 49 has a slotted socket 54 in itsouter end so that an adjusting tool may be inserted to rotate the shaft 49. Thus as shaft 49 is rotated it acts through the gear 5| to rotate the other shaft 59 so that the coil 3 may be moved toward or away from the associated coil I5 for longitudinal adjustment.

As pointed out in connection with the circuit diagram, an electrostatic shield I1 is positioned between the coils |5l6, this shield being made up of a thin sheet of copper slotted to provide a plurality of ribbons 54 which are all twisted to an angle of about 45. The ribbon strips thus are all connected at one end, that is at the top, by the supporting portion 55, the lower ends being separated and being freely suspended out of contact with any other metal elements by Virtue of the twisting of the individual ribbons. This shield I1 is suspendedbetween ametallic shielding; plate. 56; which: has. a circular. opening 5ftof substantially the diameterv of the coils :16.. Thus: the shield. I] ismounted. fixed relativeto the stationary-coil. l5, andthe adjustable coil; I6 is movable toward andaway. therefrom. 5

The inductance or choke coils 3233.of the filter circuit are each mounted ina separateshielde ed. compartment being; surrounded by. the-metal walls. 'I'hecoils 32 33 are supported upon corresponding wooden posts 5859 respectively 1o which in turn are mounted upon the metal partition Bllwhich separates the two compartments. The filter condensers -46 are shown mounted in, the compartment with the first filter section choke 32 and. grounded to the wall of the. metal 15. container at 6]..

Thus the. balancing and. filter circuit. apparatusis housed in a shielded container which. is. set within the walls of the metal shielding container for the radio receivingset R. In the views of Figs. 20;,

6 to 9, the shielded container is shown with. an open top, but. it is to. be understood. that; when placed in the shielding canof the radio receiving set R, a metal shielding coveris placed over the top.. Also, if the shielding container for the ap-ZB;

paratus which isreferred to is mountedas a separate unit, the top is. enclosed by a suitablev metal cover.

Although we have described our invention in its preferred embodiments, it is understood that we .80. are not limited thereby, but limit the invention only by the scope of the appended claims- We claim:

1. In an interference eliminator for-radio systems, means for balancing out local electrical. in- 86 terference noises in said radio receiving system, said means including. a balancing unit comprising a pair of inductively coupled coils, means for varyingthe transfer of energy between said coils, and an interposed electrostatic shield, said shield-4o, comprising a plurality of fingers connected atone end but each free over the-entire length. of the finger and twisted at the connecting end. of. the fingertostand at an angle to said connection por tion to provide a clearance between adjacent in,- dividualv fingers.

2. In a. radio receiver. system for. mounting. in an automobile-including, a. receiver circuit having an input impedance, adjacent a source of interference voltage in said automobile, a signal an- 6.0;. tenna circuit, means for eliminating said inter.- ference voltage in the receiver system, saidmeans including in combination a lead extending from a metal medium carrying interference voltage radiated from said automobile with said lead coupled to the. signal antenna circuit in such a man.- ner as to more eifectively remove therefrom. the interference voltage over one portion of the received frequency spectrum and a shielded filter inseriesconnection with the antenna circuitand the input. impedance to the receiver for more effectively removing the other portion of the interference voltage in the received frequency spectrum whereby. together to eliminate the interference voltage in said radio-receiver system.

3. Adevice adapted to be connected into a radio receiver system for eliminating interference over the received frequency spectrum in said radio receiver. system including a signal antenna, and a radio receiver having an input impedance, said 7,01 device comprising in combination a balancing portion in series connection with the signal antenna. and a. shielded filter in series withthe balancing portion. andthe. input impedance of" the mam. receiver. with. said, balancing portion; and 7.5

Ill

'said' filter each substantially eliminating some interference which the other alone would not completely eliminate, and said balancing portion and filter cooperating to substantially eliminate said interference in said radio system.

H 4. A device adapted to be connected into a radio receiver system for eliminating interference over the receivedfrequency spectrum in said radio receiver system including a signal antenna and a "radio receiver having an input impedance, said device comprising in combination a balancing unit including a pair of mutually coupled inductance .coils with the primary coil connected to a carrier of. the interference voltage and ground, and the secondary in series connection with the signal antenna, and a filter unit including a plurality of series connected inductance coils and a shunt condenser in series connection with the balancing unit and input impedance to the radio receiver, with'said balancing unit and said filter unit each substantially eliminating some interference which the other alone would not completely eliminate, and said units cooperating to substantially eliminate said interference in said radio'system.

5. In a radio receiver system for conveyances powered by an internal combustion engine, a signal antenna,.and a radio receiver having an input impedance means for eliminating interference in said radio receiver occasioned by electrical disturbances from the ignition system of said engine, .said means including in combination a shielded filter unit in series connection with the signal antenna and the input impedance of the radio receiver, and a balancing unit interposed between said antenna and said filter, and in series connection therewith, with one of said units most effective in removing interference over one portion of the received. frequency spectrum and the other unit most effective-in removing interference over the other portion of the received frequency spectrum whereby to remove said interference from the radio receiver system.

6. In a radio receiver system, a signal antenna circuit, a radio receiver including an input impedance, alocal source of electrical interference, means for removing interference voltage from said antenna circuit including in combination a balancing unit in series with said antenna having means for utilizing interference voltage substantially free from signal energy existing on a metal member common to said source to balance out interference, and a filter-unit in series with said balancing unit and the input impedance of the receiver, with said balancing unit and said filter unit each substantially eliminating some interference which the other alone would not: completely eliminate, and said units cooperating to substantially eliminate said interference in said radio system.

7. In a. radio receiver system for automobiles, a signal antenna, a radio receiver having a plurality of tube stages including a first detector, means for eliminating in said receiver system electrical interference from said automobile, said means comprising a combination balancing and filter unit connected into said receiver circuit after the signal antenna and before the first detector and in series with said signal antenna, a lead extending from a metal portion on the automobile to the balancing portion of the unit for carrying interference voltage from said metal portion to said balancer whereby to utilize said interference voltage for balancing out interference in the received frequency'spectrum, with saidbalancing portion and said filter portion each insufiicient in itself to satisfactorily eliminate interference in said receiver system, but both together operating to substantially eliminate the same.

8. In a radio receiver system for automobiles,

a signal antenna, a radio receiver including a first detector stage, means for eliminating in said receiver system electrical interference from said automobile, said means comprising a combination balancing and filter unit connected into said receiver after the signal antenna and before the first detector, said balancer portion of said unit including a pair of mutually coupled inductance coils with one of said coils in series with said antenna, a lead extending from a metal portion on the automobile to the other of said coils for carrying interference voltage from said metal portion to the balancer portion, an electrostatic shield interposed between said coils, with said filter portion connected in series with the first of said coils in the balancer portion and including a plurality of series connected inductance coils and a shunt condenser, said balancer portion and filter portion each in itself being insufiicient to satisfactorily eliminate said interference in said receiver, but both together operating to substantially eliminate the same.

9. A radio receiver system for a mobile craft mounted near an ignition system for said craft as a source of electrical interference with said radio receiver system, with said ignition system transmitting said interference currents to metal portions of the craft itself to be found as standing wave voltage thereon, and means for taking said standing wave voltage at a single point 35 on a single metal portion of the mobile craft and utilizing the same to eliminate interference in the radio receiver system.

10. A radio receiver system for automobiles, an ignition system for said automobiles as a source of electrical interference, a throttle rod for said automobile carrying a standing wave voltage from said interference source, means for eliminating interference in the radio receiver system including a balancing unit in series connection with the signal antenna of the receiver system, and a lead extending from said throttle rod to said balancing unit to transmit said standing wave voltage thereto.

11. A device for eliminating local electrical interference in a radio receiver system, said device comprising a pair of mutually coupled inductance coils, each coil consisting of a double helix shaped coil compressed to a substantially fiat double layer member for mounting in a shallow insulating casing, an electrostatic shield between said coils, with one of said coils in series connection with the signal antenna for the radio receiver system, and means for carrying interference voltage substantially free from signal voltage to the other of said coils to inductively buck out corresponding interference voltage in the radio receiver circuit.

12. A device for eliminating local electrical interference in a radio receiver system, said device comprising a pair of mutually coupled inductance coils, each coil consisting of a double helix shaped coil compressed to a substantially fiat double layer member for mounting in a shallow insulating casing, an electrostatic shield between said coils comprising a one-piece metal member having a plurality of parallel fingers extending from one end and twisted to stand at an angle to said end to provide a clearance between adjacent fingers, with one of said inductance coils in series connection with the signal antenna for the receiver system, and means for carrying interference voltage substantially free from signal voltage to the other of said inductance coils to inductively buck out corresponding interference voltage in the radio receiver circuit.

13. A'unit for eliminating high frequency interference adapted to be connected into a radio receiver system having a signal antenna and radio receiver, said unit comprising a balancing portion and a filter portion in series therewith and shielding means for said portions, and means connected with the balancing portion for conveying interference voltage to the unit substantially free from signal energy to inductively buck interference voltage received by said signal antenna, with said balancing portion and said filter portion co-acting but each insufiicient in itself to satisfactorily eliminate said interference in said receiver system and both together operating to substantially eliminate the same.

14. A device for eliminating high frequency interference adapted to be connected into a radio receiver system having a signal antenna and radio receiver, said device comprising a balancing portion and a filter portion in series therewith and shielding means for said portions, with said filter portion including an inductance coil and a condenser, a wire lead connected to the balancing portion for carrying interference voltage to the device substantially free from signal energy to inductively buck interference voltage received by said signal antenna, with said balancing portion and said filter portion each insufficient in itself to satisfactorily eliminate said interference in said receiver system but both together operating to substantially eliminate the same.

15. In a radio receiver system for conveyances powered by an internal combustion engine, said system including a signal antenna, and a radio receiver, means for eliminating in said receiver system electrical interference from said engine, said means comprising a shielded unit connected between said signal antenna and said radio receiver including a balancing portion having a pair of inductance coils with one of said coils receiving signal energy and interference voltage from the signal antenna, and means for feeding interference voltage to the other of said coils to balance the interference in said first coil, and a filter portion in series connection with the balancing portion, including an inductance coil and a condenser; with each of said portions insufficient in itself to satisfactorily eliminate interference in said receiver system, but both together operating to substantially eliminate the same.

16. An interference eliminator adapted to be connected into a radio receiver system for mounting on a mobile craft powered by an internal combustion engine and having an ignition system as a source of electrical disturbance for said radio receiver system, said receiver system including a signal antenna and radio receiver, said interference eliminator comprising a balancing portion including a pair of coupled inductance coils with one of said coils connected in series with the signal antenna and receiving signal energy and interference voltage therefrom, and means for feeding interference voltage substantially free from signal energy to the other of said coils to inductively buck the interference voltage in said first coil, a filter portion in series connection with the balancing portion having a plurality of inductance coils and a shunt condenser, and shielding means for said portions, with each of said portions of said eliminator insumcient in itself to satisfactorily eliminate interference in 5 said receiver system, but both together operating to substantially eliminate the same.

17. A radio receiver system for a mobile craft mounted near an ignition system for said craft as a source of electrical interference with said 10- said interference in the radio receiver system. 20

18. In a radio receiver system including a signal antenna and a radio receiver including the usual detector stage, with said receiver system adapted for mounting in an automobile having an ignition system as a source of electrical inter- 25 ference for said receiver system, and transmitting said interference to the metal parts of the automobile to be carried as standing wave voltages thereon, means for eliminating said interference from said radio receiver system including a bal- 30 ancing unit comprising a pair of coupled inductance coils having an electrostatic shield interposed with one of said coils in series connection with the signal antenna and connected into the receiver circuit, before the first detector, and a Single lead from a single point on a metal part of the automobile to the other coil to carry the standing wave voltage from said metal part to said balancing unit for balancing out ignition interference received by said signal antenna. 19. A device adapted to be connected into a radio receiver system for eliminating high frequency ignition interference therefrom with said radio receiver system mounted on a mobile craft powered by an internal combustion engine having an ignition system, with said ignition system transmitting interference voltage to metal parts of said mobile craft itself to stand as standing wave voltages thereon, said device comprising shielding means carrying a primary inductance coil and a secondary inductance coil variably coupled therewith and having an interposed electrostatic shield, with said secondary in series connection with the usual signal antenna for the radio receiver system and a lead extending from a metal part of the mobile craft itself to the primary coil to carry standing wave voltage thereto to be utilized in said device to balance out said ignition interference voltage.

20. An interference eliminator adapted to be connected into a radio receiver system for mounting on a mobile craft powered by an internal combustion engine and having an ignition system as a source of electrical disturbance for said radio receiver system, said receiver system including a signal antenna and radio receiver, said interference eliminator comprising a primary inductance coil and a secondary inductance coil variably coupled thereto, with a screw adjustment for varying said coupling, and shielding means for said coils, an electromagnetic shield interposed between said coils, with said secondary coil connected in series with the signal antenna and receiving signal energy and interference voltage therefrom and a single lead extending from a metal portion on the mobile craft to the primary coil to carry interference voltage thereto substantially free from signal energy to inductively buck out the interference voltage in the secondary coil.

21. In a radio receiver system including an antenna and a radio receiver, means for removing electrical interference in said system, with said means adapted for connecting therein between said antenna and said radio receiver, said means including in combination an interference balancing portion comprising a pair of coupled inductance coils with adjusting means for varying the coupling between said coils, and a filter portion including an inductance in series connection with said balancing portion, and including a shunt capacitance with said balancing portion and said filter portion of said removing means each substantially eliminating interference which the other alone would not completely remove. 

